External library linking problem

Hi there guys, I am trying to link libxml++ which is a lib for parsing HTML and XML. I've installed the lib, however when I go to Qt and try to link the lib I get all sorts of linker error. I tried linking the lib by right clicking on the project and choosing "add library", which seems to work just fine however when I try to use any of the lib objects Qt tells me that the header file does not exist.

Show your *.pro file.
Which IDE are you using?